The present invention relates to a semiconductor self-pulsating laser diode and to a method for causing a semiconductor laser diode to output a plurality of light pulses in sequential light pulse cycles.
Semiconductor self-pulsating laser diodes are known. Commonly, such self-pulsating laser diodes output pulsed light of wavelengths of approximately 800 nm. Such laser diodes are commonly used for reading data from a disc, for example, a CD disc. It is widely believed that the self-pulsating behaviour of semiconductor laser diodes results from the provision of saturable absorber regions positioned adjacent an active wave guiding region of the laser diode such that light propagating in the active wave guiding region overlaps the saturable absorber region. The saturable absorber regions are formed by materials which have an intensity dependent absorption coefficient. As the active region is excited, the saturable absorber region absorbs light generated in the active wave guiding region, which in turn generates charge carriers in the saturable absorber region. The build-up of charge carriers in the saturable absorber region reduces the absorption coefficient of the material of the saturable absorber region, and thus reduces its capacity to absorb additional photons at that wavelength. Further excitation of the active wave guiding region causes the saturable absorber region to saturate, from which the term xe2x80x9csaturable absorberxe2x80x9d is derived. At saturation the absorber material of the saturable absorber region has a decreased loss, and thus the lasing condition for the laser diode is met and lasing light is emitted from the laser diode. The resulting emission of light from the laser diode depletes the charge carriers in the active region until lasing stops. Charge carriers generated in the saturable absorber region diffuse out of this region and return the absorber material to its original high absorption state, aiding quenching of the lasing emission and commencing the next cycle. In this manner a series of pulses of light are emitted from the laser diode in sequential cycles at a repetition rate which is predominantly determined by the carrier dynamics of the laser diode.
Many such self-pulsating laser diodes are described in the patent literature. U.S. Pat. Nos. 5,416,790 and 5,610,096 (both assigned to Sanyo Electric Co. Ltd.) disclose AlGaAs semiconductor lasers comprising a saturable optical absorbing layer having a band gap energy substantially equal to the energy corresponding to the lasing wavelength. U.S. Pat. No. 5,581,570 (assigned to Mitsubishi Denki Kabushiki Kaishi) discloses a semiconductor laser device comprising a saturable absorption region the function of which is the production of enhanced and pulsation oscillation at high power light output. In R. C. P. Hoskins, T. G. van de Roer, C. J. van der Poel, H. P. M. Ambrosius: Applied Physics Letters 67, 1343, (1995), Hoskins et al teach that self-pulsation can be induced in broad area AlGaAs diode lasers by including an extra GaAs layer functioning as a saturable absorber.
In J. Buus: IEEE Journal of Quantum Electronics 19,953, (1983), Buus has suggested that self-focusing, caused by the dependence of the refractive index on the carrier density, may contribute to self-pulsation in GaAs/GaAlAs devices which have no built-in wave guide. Buus discusses devices with relatively wide lasing stripes defined by the current injection regions which guide light by virtue of confining the gain to a stripe impressed along the light propagation axis of the laser. Any index guiding effects that may occur are fortuitous or incidental due to optically, photoelastically and thermally induced phenomena.
Considerable efforts have been made to develop a self-pulsating laser which would emit light at wavelengths shorter than 800 nm in order to satisfy demand for higher storage capacity capabilities. Such lasers are made from materials well-known to those skilled in the art and comprise suitable combinations of elements such as Indium, Gallium, Arsenic, Phosphorous, Aluminium, Nitrogen Cadmium, Zinc, Sulphur and Selenium for short wavelength (less than 700 nm) operation. Additionally, telecommunications applications could utilise robust self-pulsating laser devices made from these and other well-known elements which emit around longer wavelengths such as 0.98 xcexcm, 1.3 xcexcm or 1.5 xcexcm.
Applying the saturable absorber approach to materials which emit light at 650 nm, Kidoguchi et al [I. Kidoguchi, H. adachi, T. Fukuhisa, M. Mannoh, A. Takamori: Applied Physics Letters 68, 3543, (1996)] teach that self-pulsating AlGaInP lasers emitting light at a wavelength in the order of 650 nm may be fabricated by adopting a structure which has a highly doped saturable absorbing layer. Again using the saturable absorber approach U.S. Pat. No. 5,850,411 (assigned to SDL Inc.) discloses an AlGaInP/GaAs laser diode in which the active region is made up of quantum wells that are less than 5 nm thick such that quantum confinement of the charge carriers becomes significant. This facilitates operation with light emission of wavelength in the order of 620-650 nm. It is taught therein that self-pulsation may be obtained by the inclusion into such structures of a saturable absorber layer proximate to the active region.
Other devices emitting at 650 nm based on the incorporation of saturable absorbers are known. However, 650 nm lasers incorporating saturable absorbers have a number of disadvantages. The major disadvantage is the increased threshold current required to achieve lasing and to realise self-pulsation. This increase in threshold current arises from the increased loss introduced into the laser cavity by inclusion of the saturable absorption layer. An increase in the threshold current leads to an undesirable increase in heating in the device, which reduces the gain available in the laser due to its decrease with increasing temperature. Furthermore, in quantum well devices designed to emit at around 650 nm, or 1.3 to 1.5 xcexcm for that matter, an increase in the threshold current required to operate the device causes an increase in leakage of charge carriers over the hetero-barriers of the structure. This in turn has a detrimental impact on the temperature and self-pulsation properties of the device.
There is therefore a need for a self-pulsating laser diode which overcomes the disadvantages of known self-pulsating laser diodes, and there is also a need for a method for causing a laser diode to output light pulses.
The present invention is directed towards providing such a self-pulsating laser and such a method.
According to the invention there is provided a semiconductor self-pulsating laser diode comprising a wave guiding layer, wherein the laser diode is configured so that when the laser diode is pumped,
(a) an active wave guiding region is defined in the wave guiding layer, the active wave guiding region comprising a pulsed light generating region in which pulsed light is guided during respective sequential light pulse cycles, the pulse light generating region extending longitudinally in the direction of pulsed light propagation, and an adjacent light propagating region in which light is propagated, and
(b) during each light pulse cycle the carrier density profile across the active wave guiding region progressively varies such that initially the carrier density in the pulse light generating region rises relative to the carrier density in the light propagating region until the difference between the refractive index of the pulse light generating region and the refractive index of the light propagating region is at its greatest, and the carrier density of the pulse light generating region reaches its lasing threshold value, thus causing lasing to commence in the active wave guiding region, and the lasing in the pulse light generating region progressively reduces the carrier density therein, which in turn progressively reduces the relative difference between the refractive index of the pulse light generating region and the light propagating region until the refractive index of the pulse light generating region approaches the refractive index of the light propagating region, thereby increasing guiding of the lasing light into the pulse light generation region for emission of the light pulse therefrom, at which stage the carrier density of the active wave guiding region falls below its lasing threshold value extinguishing the lasing light, and the next light pulse cycle commences.
In one embodiment of the invention the laser diode is configured so that the progressive reduction in the carrier density of the pulse light generating region after lasing commences in the active wave guiding region is further facilitated by photopumping and charge carrier diffusion from the pulse light generating region to the light propagating region.
Preferably, the laser diode is configured so that during pumping of the laser the refractive index of the light propagating region is higher than the refractive index of the adjacent wave guiding layer. Advantageously, the amount by which the refractive index of the pulse light generating region is less than the refractive index of the light propagating region at the beginning of each light pulse cycle is substantially similar to the amount by which the refractive index of the light propagating region is higher than the refractive index of the adjacent wave guiding layer. Preferably, the transition of the refractive indices between the higher refractive index of the light propagating region and the lower refractive index of the wave guiding layer during pumping of the laser diode is substantially a step transition.
In one embodiment of the invention the light propagating region is located in the wave guiding layer on respective opposite sides of the pulse light generating region, and extends parallel to the pulse light generating region. Preferably, the light propagating region surrounds the pulse light generating region.
In another embodiment of the invention the laser diode is configured by providing a current blocking layer in the laser diode, the current blocking layer defining a current passageway for the passage of a pumping current therethrough and defining the active wave guiding region in the wave guiding layer. Preferably, the current blocking layer extends parallel to the wave guiding layer and defines an elongated slot extending parallel to the direction of light propagation in the pulse light generating region for defining the current passageway.
In one embodiment of the invention the wave guiding layer is sandwiched between a pair of cladding layers. Preferably, the respective cladding layers extend parallel to the wave guiding layer.
In another embodiment of the invention the current blocking layer is located in one of the cladding layers.
In a further embodiment of the invention the current blocking layer is shaped so that the pumping current defines the active wave guiding region. Preferably, the current blocking layer is shaped to define the pulse light generating region and the light propagating region. Advantageously, the current blocking layer is shaped adjacent the current passageway for defining the active wave guiding region.
In one embodiment of the invention elongated recesses for defining the active wave guiding region are formed on respective opposite edges of the current blocking layer which define the current passageway. Preferably, the recesses formed in the respective opposite edges of the current blocking layer defining the current passageway are formed in the respective edges and a major surface of the current blocking layer which is closest to the wave guiding layer.
In another embodiment of the invention a pair of parallel spaced apart barrier means are located in the wave guiding layer extending parallel to the direction of light propagation in the pulse light generating region, the barrier means defining the active wave guiding region therebetween in a lateral direction perpendicular to the direction of light propagation in the pulse light generating region. Preferably, the respective barrier means co-operate with the current blocking layer for defining the active wave guiding region.
In another embodiment of the invention the laser diode is configured by the provision of a pair of parallel spaced apart channels located in one of the cladding layers adjacent the pulse light generating region, the respective channels extending parallel to the direction of light propagation in the pulse light generating region, and containing material of refractive index higher than the material of the cladding layer for defining the active wave guiding region. Preferably, the respective channels are located in the cladding layer opposite the cladding layer in which the current blocking layer is provided. Advantageously, the refractive index of the material contained in the respective channels is similar to that of the wave guiding layer. Ideally, the respective channels co-operate with the current blocking layer for defining the active wave guiding region.
In one embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 1,700 nm.
In another embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 1,550 nm.
In a further embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 1,300 nm.
In a still further embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 980 nm.
In a still further embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 700 nm.
In a still further embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 650 nm.
In a still further embodiment of the invention the material of the wave guiding layer is selected for outputting light pulses of wavelength up to 600 nm.
Additionally, the invention provides a method for causing a laser diode to output a plurality of light pulses in respective sequential light pulse cycles, the laser diode comprising a wave guiding layer, and the method comprising the steps of configuring the laser diode so that when the laser diode is pumped,
(a) an active wave guiding region is defined in the wave guiding layer, the active wave guiding region comprising a pulsed light generating region in which pulsed light is guided during respective sequential light pulse cycles, the pulse light generating region extending longitudinally in the direction of pulsed light propagation, and an adjacent light propagating region in which light is propagated, and
(b) during each light pulse cycle the carrier density profile across the active wave guiding region progressively varies such that initially the carrier density in the pulse light generating region rises relative to the carrier density in the light propagating region until the difference between the refractive index of the pulse light generating region and the refractive index of the light propagating region is at its greatest, and the carrier density of the pulse light generating region reaches its lasing threshold value, thus causing lasing to commence in the active wave guiding region, and the lasing in the pulse light generating region progressively reduces the carrier density therein, which in turn progressively reduces the relative difference between the refractive index of the pulse light generating region and the light propagating region until the refractive index of the pulse light generating region approaches the refractive index of the light propagating region, thereby increasing guiding of the lasing light into the pulse light generation region for emission of the light pulse therefrom, at which stage the carrier density of the active wave guiding region falls below its lasing threshold value extinguishing the lasing light, and the next light pulse cycle commences.
The advantages of the invention are many. A particularly important advantage of the self-pulsating laser diode according to the invention is that it enables relatively simple, inexpensive and efficient self-pulsating lasers which emit light of wavelengths less than 800 nm to be provided. In particular, the self-pulsating laser according to the invention can be provided to produce pulsating light of wavelengths of approximately 650 nm, which is the optimum wavelength for reading data on CDs, CD-ROMs, DVDs, and other such optical discs.
Furthermore, it has been found that the laser diode is particularly suitable for providing pulsed light of wavelengths up to 1,700 nm, and indeed, the laser diode according to the invention is particularly suitable for providing pulsed light of 600 nm, 650 nm, 700 nm, 980 nm, 1,300 nm and 1,550 nm, as well as 1,700 nm.